1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method for selecting, based on an image of the surface of the moon or planets, a landing spot for a landing craft such as an unmanned exploration vehicle.
2. Description of the Related Art
When an unmanned explorer or other type of unmanned space craft lands on the moon or planets, avoiding surface obstacles at the landing site is essential to making a safe landing. This is particularly important when an unmanned explorer or other unmanned craft attempts an autonomous landing in an area for which detailed topographical data is not available. This is because the landing craft could topple over if a leg of the landing craft, for example, contacts a large obstacle at the landing site.
Devices for identifying obstacles that could obstruct a safe landing as a means of identifying a safe landing site have been proposed in the literature. Such devices typically capture an aerial image of a potential landing area using an optical, laser or, millimeter waves, or other method, and detect obstacles based on the resulting image data.
A landing site identification device as taught in Japanese Patent Laid-Open Publication No. 8-181976, for example, uses a camera to photograph a potential landing site, identifies shadows based on the resulting image data, and recognizes the detected shadows as areas where landing is not possible (unlandable areas). When identification of unlandable areas is completed, an area where landing is possible (landable area) is detected and the center of the identified landable area is defined as the landing site. The landable area in this case is determined by seeking a site in which a landing site error circle, determined by the guidance precision of the landing craft, does not overlap an unlandable area.
Another obstacle detection method is taught in the paper "An examination of obstacle detection and avoidance methods for lunar surface landing" by Ogasawara et al. (40th Joint Symposium of Space Science and Technology, 2A05, 1997; in Japanese).
FIG. 10 is a figure included in the above paper by Ogasawara et al. FIG. 10A shows an image from a photograph of the lunar surface, the top image covering an area 1800.times.1800 m, and the lower image showing a 600.times.600 m section from the middle of the upper image. The top and bottom 3D graphs shown in FIG. 10B correspond to the top and bottom images shown in FIG. 10A. The graphs were obtained by dividing the images in FIG. 10A into pixel matrices, and counting the number of pixels with brightness below a specified threshold value in each pixel matrix. The Z-axis of each graph shows the pixel count in each matrix, and the X and Y axes represent the X and Y coordinate positions of each pixel matrix in the corresponding image.
The obstacle detection method taught in the above paper is designed specifically for detecting craters with a diameter of 2 m or more. When the pixel count in a given matrix exceeds a specific threshold value, the corresponding pixel matrix is determined to belong to a shadow area. Crater location and size are then estimated using the resulting shadow area information, solar declination, and crater model.
As described above, a typical conventional image processing device for enabling a safe lunar or planetary landing determines the presence of any obstacles based solely on information about areas containing shadows with a low brightness level. This method can therefore be effectively used to detect anticipated obstructions such as rocks projecting from flat surfaces, and specific obstacles, such as craters, that match the shape of defined models. It is difficult, however, to detect unanticipated obstacles and obstacles that are hard to model, including, for example, overlapping craters that do not conform to the crater model.
There is therefore a need for an image processing apparatus and image processing method whereby all types of surface formations and objects that may be an obstacle to a safe landing can be avoided during landing by a landing craft based on an image of the area around a landing site.